The problem of shape control of composite laminated smart structures with piezoelectric patches placed at optimal location is considered in this thesis. Laminated plate structures with piezoelectric patches for shape control applications are modeled using a shear deformable plate formulation by including the piezoelectric layers into the plate substrate. A composite plate finite element model is also developed for composite plates with self-sensing actuators. Non-linear hysteresis models for piezoelectric materials are presented and discussed. Numerical simulation of composite plate structures with piezoelectric actuators is conducted and presented. The optimization problem of finding the optimal location of actuators using a linear quadratic control algorithm is done and the results are discussed. Static shape control strategies are also discussed.

The problem of shape control of composite laminated smart structures with piezoelectric patches placed at optimal location is considered in this thesis. Laminated plate structures with piezoelectric patches for shape control applications are modeled using a shear deformable plate formulation by including the piezoelectric layers into the plate substrate. A composite plate finite element model is also developed for composite plates with self-sensing actuators. Non-linear hysteresis models for piezoelectric materials are presented and discussed. Numerical simulation of composite plate structures with piezoelectric actuators is conducted and presented. The optimization problem of finding the optimal location of actuators using a linear quadratic control algorithm is done and the results are discussed. Static shape control strategies are also discussed.

Indiana University-Purdue University Indianapolis (IUPUI) / The expansion of the applications of direct band gap semiconductor nanocrystals (NCs) has been a result of the control colloidal synthetic methods offer on the optoelectronic properties. These properties are readily controlled by the surface chemistry and even a small change in the surface passivating ligand can show profound effects. Furthermore, the choice of surface passivating ligand also impacts the NC shape evolution, which in turn influence the surface area, quantum yield, and charge transport properties that are critical to optimize device fabrication. In this dissertation, the unique aspects of surface chemistry that control both NC shape evolution and optoelectronic properties are investigated. We began by investigating how surface chemistry controls the shape evolution of methyl ammonium lead bromide (CH3NH3PbBr3) perovskite NCs. In addition to the surface passivating ligand, the reaction temperature and solvent system were also examined. Through a series of control experiments, the critical parameter for the formation of quantum wires (QWs) was found to be the presence of a long chain acid, while the quantum platelets (QPLs) required a long chain amine and chlorinated solvent, and quantum cube (QC) formation was kinetically driven. The higher ordered stacking of the QPLs and bundling of the QWs was also found to be controlled by surface ligand chemistry. Next we further examined how surface chemistry impacts shape evolution, but in the system of metal chalcogenide NCs. We developed a versatile, low temperature, and gram scale synthesis of QWs, QPLs, and quantum rods (QRs) using both cadmium and zinc as metal precursors and sulfur and selenium as chalcogenide precursors. Through systematic investigation of both the surface chemistry and reaction progression, the growth and formation mechanism was also determined. The 1D QW growth required a long chain amine while the QPLs required the presence of both a long and short chain amine to drive 2D growth. Finally, the QRs would found to be a kinetically-controlled process. Ultrasmall semiconductor NCs are known to possess high surface to volume ratios and therefore even a minute change in surface chemistry will have a significant impact on the optoelectronic properties. Our investigation focused on (CdSe)34 NCs, and how exchanging native amine ligands with various chalcogenol based ligands influences these properties. These NCs lie in the strong confinement regime and therefore have a higher probability of undergoing exciton delocalization, resulting in red shifts of the first excitonic peak and reduction of the optical band gap. Additionally, we examined different characteristics of the ligand (level of conjugation, electron withdrawing or donating nature of para-substitution, binding mode and head group) to examine how these parameters impact exciton delocalization. We observed the highest shift in the optical band gap (of 650 meV) after exchanging the native amine ligands with pyrene dithiocarbamate. Through this investigation it was determined that ligand characteristics (specifically conjugation and binding mode) have significant influence in the proposed hole delocalization. Finally, we continued the investigation of how surface chemistry controls optoelectronic properties of ultrasmall NCs, but expand our work to those of methyl ammonium lead halide. We developed a low temperature and colloidal synthesis of white-light emitting NCs with a diameter of 1.5 nm. Through precise manipulation of the surface halide ions, it was possible to tailor the emission to match that of nearly pure white light.

Thesis advisor: Chia-Kuang F. Tsung / Thesis advisor: Dunwei Wang / In order to understand the lattice strain effect and its relationship to size, shape, composition, and catalytic performance, novel well-defined nanocrystal archetypes were designed and synthesized by taking advantage of wet chemical, seed-mediated (mild) reduction routes developed by our lab. First, the current synthesis challenges are addressed in creating smaller monometallic shape-controlled metal nanocrystals, and novel cuboctopods via a hybrid nanoparticle stabilizer. A look at the relationship between lattice strain and morphology is then shown in a single-component system, where still new features have been observed for the first time by the traditional technique of powder x-ray diffraction. Synthesis methods for differently strained Pd surfaces are described and catalysis by these surfaces is discussed. Finally, studies of the synthesis, characterization, electrocatalytic activity, and restructuring of novel and more sophisticated strained architectures are presented: core-island-shell nanocrystals, phase-segregated nanoboxes, island nanoframeworks, and core-sandwich-shell nanoparticles. Lattice strain and composition effects were studied in carbon monoxide, small alcohol, and formic acid electrocatalytic oxidations as well as in oxygen reduction, the latter of which, governs the commercial viability of automotive fuel cells, a sustainable energy and zero-emission technology. Here it is demonstrated how a tunable thickness of Ni sandwich layers can be used to improve catalytic performance by increasing lattice strain on the Pt surface. The sandwich archetype offers a new platform for the investigation of lattice strain and could be a promising, industrially relevant, catalyst design concept, to help address the need for a more sustainable energy future. The results help paint a new picture of catalysis by metal nanocrystals; one which brings lattice strain to the forefront of the discussion, as an important parameter for further study and for use in developing higher-performing catalysts. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Piezoelectric actuators are well suited for high precision mechanical and electrical engineering applications. However, its performance in regulator configurations has been limited due to hysteresis. The hysteresis in these actuators means that multiple input states can result in the same output, which introduces a further design variable (initial state) in the regulation problem. It is proposed that certain initial states result in better regulation performance based on the structure of the Preisach model. These initial states are called “neutral states”. In this thesis, hysteresis and piezoelectric actuators are introduced as background information. The Preisach model is used in this work to describe the hysteresis behaviour of a customized shape control unit SS15 due to its convenient general structure and ability to model hysteresis. The representation tests are performed and a Preisach model is subsequently constructed and verified by comparing simulation and experimental results to ensure that the hysteresis inherent in the piezoceramic actuators of the SS15 is suitably described by this model. In order to evaluate the regulation performance for a given desired output, uniformly-distributed noise is injected at the input side of the SS15 in open- and closed-loop tests. It is demonstrated, by both simulation and experimental results, that the system output drifts less when it starts from the neutral state in open-loop tests. A PI regulator is implemented in the closed-loop tests. When the system is driven from the neutral state, both simulation and experimental results demonstrate that the system requires less control effort for closed-loop regulation.

Piezoelectric actuators are well suited for high precision mechanical and electrical engineering applications. However, its performance in regulator configurations has been limited due to hysteresis. The hysteresis in these actuators means that multiple input states can result in the same output, which introduces a further design variable (initial state) in the regulation problem. It is proposed that certain initial states result in better regulation performance based on the structure of the Preisach model. These initial states are called “neutral states”. In this thesis, hysteresis and piezoelectric actuators are introduced as background information. The Preisach model is used in this work to describe the hysteresis behaviour of a customized shape control unit SS15 due to its convenient general structure and ability to model hysteresis. The representation tests are performed and a Preisach model is subsequently constructed and verified by comparing simulation and experimental results to ensure that the hysteresis inherent in the piezoceramic actuators of the SS15 is suitably described by this model. In order to evaluate the regulation performance for a given desired output, uniformly-distributed noise is injected at the input side of the SS15 in open- and closed-loop tests. It is demonstrated, by both simulation and experimental results, that the system output drifts less when it starts from the neutral state in open-loop tests. A PI regulator is implemented in the closed-loop tests. When the system is driven from the neutral state, both simulation and experimental results demonstrate that the system requires less control effort for closed-loop regulation.

Composite materials are increasingly used in aerospace, underwater, and automotive structures. Their use in structural applications is dictated by the outstanding strength and stiffness while being lightweight in addition to their flexibility in tailoring the desired performance in the design of structures. The present study focuses on the failure analysis and shape control of smart composite laminates under coupled hygrothermal, electric and mechanical stimuli. A linear thermo-electro-elastic constitutive model for transversely isotropic materials is used for each ply in the composite laminates. The first-ply failure and ultimate laminate failure criteria of composite laminates are used to predict the failure stress and mode of the composite laminate where we incorporate various commonly known macroscopic failure criteria including Tsai-Hill, Tsai Wu, maximum stress and maximum strain for each lamina. We study the use of piezoelectric materials such as lead zirconate titanate (PZT) and piezoelectric fiber composites as actuators for controlling deformation in composite laminates; this study focuses on bending deformation. The purpose is to minimize unwanted deformation, such as the one due to hygrothermal effect, by applying counter deformation to avoid failure in such composite laminates. In addition, analysis based on the Classical Laminate Theory (CLT) is performed for Carbon/Epoxy (AS4/3501-6) thin laminate with stacking sequence [90/45/-45/0]s under uniaxial and biaxial in-plane loading. One of the major types of failure in smart structures is caused by debonding of the actuator from the host structure which is caused by the high stress discontinuity between the interface of the host structure and the active part. By using embedded actuators, such that the active part is incorporated into one of the layers of the composite beam during the manufacturing process, the stress concentration effect can be reduced while obtaining similar actuation values. Moreover, a control algorithm is proposed that enables the composite laminate to overcome the failure load by using piezoelectric materials where a counter electric voltage could be applied which prevents failure from occurring. Furthermore, computer software called “Hyper Composite” was developed using Action Script® and Adobe Flash® in order to perform stress and failure analysis for general composite laminates. Several carpet plots were also generated to show the interacting behavior of two independent variables such as Young’s modulus, Poisson’s ratio, shear modulus and the coefficient of thermal and moisture expansion at different percentile constitutions for the laminate different plies. This computer software is useful for estimating overall properties of smart composite laminates in designing smart composite structures.

General unsymmetric laminates exhibit large natural curvatures at room temperature. Additionally, inherent to most unsymmetric laminates is the presence of two stable configurations. Multiple configurations and stability issues arise because of the geometric nonlinearities associated with the large curvatures. The laminate can be changed from one stable configuration to the other by a simple snap-through action. This situation offers the opportunity to use shape memory alloys (SMA) attached to the laminate to generate the snap-through forces and change the shape of the laminate on command. Presented is a model which can predict SMA-induced deformations in general unsymmetric laminates and, particularly, the occurrence of the snap through. First, a methodology is developed to predict the deformations of flat general unsymmetric epoxy-matrix composite laminates as they are cooled from their elevated cure temperature. Approximations to the strain fields are used in the expression for the total potential energy, and the Rayleigh-Ritz approach is used to study equilibrium. To further study the laminate deformations, finite-element analyses are performed. Experimental results are presented which confirm the predictions of the developed theory and the finite-element analyses regarding the existence of multiple solutions and the magnitude of the deformations. Results are compared with those of several other investigators. Next, the deformation behavior of general unsymmetric laminates subjected to applied forces is studied. The principle of virtual work is used to derive the equilibrium equations relating the laminate deformations to the applied forces. By solving the equilibrium equations as a function of the force level, relations between the laminate deformations and the applied force are derived, and the force level at which the laminate changes shape is determined. Finally, an existing SMA constitutive model is implemented into the developed theory to predict the deformations of simple structures to SMA-induced forces. Experiments on a narrow aluminium plate with an externally attached SMA actuator are conducted. The experimental results show good agreement with the predictions from the developed theory. Next, the deformation behavior of general unsymmetric laminates subjected to SMA actuators is predicted using the developed theory. Experiments using SMA actuators to generate the snap through of nsymmetric laminates are conducted. Good correlation with the developed theory is obtained. / Ph. D.

Some equations of linear elasticity are developed, including those specific to certain actuator structures considered in formation theory. The invariance of the strain-energy under the transformation from rectangular to spherical coordinates is then established for use in two specific formation problems. The first problem, involving an elastic structure with a cylindrical equilibrium configuration, is formulated in two dimensions using polar coordinates. It is shown that L² controls suffice to obtain boundary displacements in H^1/2. The second problem has a spherical equilibrium configuration and utilizes the elastic equations in spherical coordinates. Results similar to those obtained in the two dimensional case are indicated for the three dimensional problem. / Ph. D.

Exciting periods of scientific research are often associated with discoveries of novel materials. Such period was brought about by the successful preparation of graphene which is a 2D allotrope of carbon with remarkable electronic, optical and mechanical properties. Functional graphene-based nanocomposites have great promise for applications in various fields such as energy conversion, opteoelectronics, solar cells, sensing, catalysis and biomedicine. Herein, microwave and laser-assisted synthetic approaches were developed for decorating graphene with various semiconductor, metallic or magnetic nanostructures of controlled size and shape. We developed a scalable microwave irradiation method for the synthesis of graphene decorated with CdSe nanocrystals of controlled size, shape and crystalline structure. The efficient quenching of photoluminescence from the CdSe nanocrystals by graphene has been explored. The results provide a new approach for exploring the size-tunable optical properties of CdSe nanocrystals supported on graphene which could have important implications for energy conversion applications. We also extended this approach to the synthesis of Au-ceria-graphene nanocomposites. The synthesis is facilely conducted at mild conditions using ethylenediamine as a solvent. Results reveal significant CO conversion percentages between 60-70% at ambient temperatures. Au nanostructures have received significant attention because of the feasibility to tune their optical properties by changing size or shape. The coupling of the photothermal effects of these Au nanostructures of controlled size and shape with GO nanosheets dispersed in water is demonstrated. Our results indicate that the enhanced photothermal energy conversion of the Au-GO suspensions could to lead to a remarkable increase in the heating efficiency of the laser-induced melting and size reduction of Au nanostructures. The Au-graphene nanocomposites are potential materials for photothermolysis, thermochemical and thermomechanical applications. We developed a facile method for decorating graphene with magnetite nanocrystals of various shapes (namely, spheres, cubes and prisms) by the microwave-assisted-reduction of iron acetylacetonate in benzyl ether. The shape control was achieved by tuning the mole ratio between the oleic acid and the oleyamine. The structural, morphological and physical properties of graphene-based nanocomposites described herein were studied using standard characterization tools such as TEM, SEM, UV-Vis and PL spectroscopy, powder X-ray diffraction, XPS and Raman spectroscopy.